Living in New York, one’s mind might be lent to wonder about the legality of street advertisement. They are literally everywhere. I’m not just talking about billboards (though their omnipresence is also very hard to dismiss), but on every barricade, construction site (New York has many) and even just on empty walls.
According to this article on Rocketboom, most street ads are totally illegal. A group called Public Ad Campaign decided to do something about it. They reclaimed visual space with white paint and put wonderful street art in place of the ads.